1. Field of the Invention
The present invention relates to controlled release fertilisers and, particularly, to fertilisers exhibiting a release of nutrients that can be delayed for a period of time after fertilisation and to processes for the preparation of such fertilisers.
2. Description of Related Art
Coated (or encapsulated) fertilizers are known to be very effective sources to provide a controlled release of nutrients for the feeding of plants. The nutrients are released at controlled rates through the fertiliser's coating resulting in a sustained feeding of plants. As a result, one application of these so-called controlled release fertilizers (CRFs) can provide the necessary nutrients for a plant that would take multiple applications of soluble fertilizers. One type of coated fertiliser in wide use is sulfur coated fertiliser, such as disclosed in U.S. Pat. No. 4,042,366, U.S. Pat. No. 4,636,242 and WO 94/29239. The release of nutrients from sulfur-coated fertilizers occurs by diffusion through imperfections in the sulfur coating and through coating breakdown. The major advantage of the sulfur coated fertilizers is their relatively low cost.
A second type of CRFs utilizes solvent applied polymer coatings. The polymeric materials applied are either thermosetting resins or thermoplastics. Examples of solvent applied thermosetting resin coated fertilizers which are currently in use are disclosed in U.S. Pat. No. 3,223,528, GB 2,202,523 and EP 0,184,869, whereas examples based on thermoplastics can be found in U.S. Pat. No. 4,019,890. Another type of encapsulated fertilizers that exhibits good controlled release properties is latex coated granular fertilizers such as those disclosed in U.S. Pat. No. 4,549,897 and U.S. Pat. No. 5,186,732. Both solvent and latex applied polymer coated fertilizers offer important benefits over sulfur-coated products concerning consistency of release rates. The majority of nutrient release is by diffusion through pores in the polymer coating, rather than release through coating imperfections.
The presence of a polymeric coating on CRFs allows for a rather uniform and consistent nutrient release, provided that the barrier properties of the polymer are sufficient. However, in general these CRFs show a high release rate of nutrients during the first period after application, which is followed by decreasing nutrient release rate during the next period. Finally, the fertiliser granules become exhausted resulting in a further decrease in release rate. In general, the accumulated nutrient release curves can be characterised mathematically by smooth quadratic (convex) curves.
Regarding CRFs which are characterised by a delayed start of the release of nutrients after application, one type is known. Multi-layered polyolefin-coated (POC) CRFs, see e.g. JP 09/241,090, have been claimed to exhibit delayed release determined by the incorporation of chemicals in the coating. The major disadvantage of these type of delayed release POC CARS is the fact that at least two coatings are required, i.e. one to give the specific release pattern and one to provide the delay. Furthermore, the presence of specific chemicals in the delayed start coating is required.